This invention relates to fluid control and, more particularly, to a tank filler valve that automatically shuts off when the tank is filled with a volatile liquid to a specified level.
It is dangerous to overfill a tank with highly volatile liquid, such as liquid petroleum gas (LPG), because of the possibility that the tank will rupture due to liquid expansion in the tank with rising temperature. For example, a tank should not be filled with propane to a level higher than 80% of its capacity. As a safety measure, tanks are currently provided with a dip tube that leads from a point at the maximum fill level within the tank to a point external to the tank. When the tank is filled to the maximum level, liquid is drawn through the tube and gives a visual signal outside the tank to the operator who is filling the tank. However, if the operator doesn't open the dip tube valve before filling the tank or inadvertently or intentionally continues to fill the tank after the visual signal is given, a dangerous overfilled condition will occur.
Mylander U.S. Pat. No. 3,363,641 discloses a flow control valve that automatically shuts off when a tank has been filled with a volatile liquid to a predetermined maximum level. The valve has a main inlet, a main outlet, and a main passage from the main inlet to the main outlet formed in a valve body. A piston chamber formed in the valve body has a valve seat in the main passage. A piston is movable back and forth through the piston chamber between a position in engagement with the valve seat to close the main passage, thereby shutting off the valve, and a position out of engagement with the valve seat to open the main passage. An O-ring seal between the piston and the piston chamber creates on the piston a first area against which fluid pressure in the main passage acts to urge the piston toward its open position. A control chamber lies in fluid communication with one end of the piston chamber. A dip tube having a small orifice leads from a point at the maximum fill level in the tank to the control chamber. The control chamber is vented to the atmosphere through another orifice substantially similar to the orifice in the dip tube. Within the control chamber the piston has a second area against which fluid from the dip tube acts to urge the piston toward its closed position. By virtue of the pair of orifices a substantially larger pressure is generated in the control chamber when liquid is fed thereto through the dip tube than when gas is. Before the tank is filled to its maximum level, the pressure of the gas fed to the control chamber by the dip tube is insufficient to overcome the force exerted on the first area of the piston due to the fluid pressure in the main passage. When the tank is filled to the maximum level, however, liquid is fed to the control chamber through the dip tube. The resulting larger pressure exerts a force on the second area of the piston larger than the force exerted on the first area thereof, and the piston moves to its closed position. The first and second areas of the piston are almost the same, so that a large increase in pressure must be developed when liquid is fed to the control chamber in order to shut off the valve.
In a commmercial version of the Mylander valve, an actuator plate, having a larger area than the first and second areas of the piston, loosely fits in the control chamber so that the pressure of the liquid from the dip tube drives the actuator plate against the second area of the piston, thereby shutting off the valve. Because of the loose fit of the actuator plate, there may be a delay of as long as eight to ten seconds from the time the liquid in the tube reaches its maximum level to the time the valve shuts off. Such delay may result in an appreciable overfilling of the tank.